Preheat oven to 350 F. Slice squash & onions, this can be done night before, store in baggies in refrigerator.
In large pan on medium-high heat, saute squash in 1 Tbsp. butter just till tender, about 2-3 min. In same saucepan, saute onions in 1 Tbsp. butter till tender, about 2-3 min. DO NOT OVER COOK squash & onions, or they will be mushy in casserole. Put squash & onions in a large mixing bowl. Let cool.
In same saucepan on medium heat, melt 2 Tbsp. butter, whisk in 2 heaping Tbsp. flour constantly whisking till flour is cooked, for about 1 minute. Reduce heat and slowly add chicken stock, whisking to prevent lumps. Add sour cream, garlic powder, sugar, salt & pepper, hot sauce and constantly stir. Cook 1-2 minute or until thickened, remove from heat.
Add 1 beaten egg and ½-¾ cup of Panko to cooled squash & onion mixture. Add the thickened chicken sauce, salt & pepper to taste. Mix thoroughly and put in casserole dish (9x7) sprayed with cooking spray. Melt 2 Tbsp. butter in same saucepan, remove from heat and mix in additional Panko (about 1 cup, enough to cover squash), put on top of squash mixture, dot with extra butter, if desired and sprinkle with Paprika (optional).
Cook @ 350 F. for 50-60 minutes. Casserole taste better the next day!